In active seismic exploration, a subterranean area of interest is typically imaged by transmitting shots from active seismic sources and receiving reflected sonic/acoustic energy at multiple sensors/receivers (i.e., geophones or hydrophones) arranged in an array. The signals received at each geophone define a trace of seismic data. Each such trace may include a number of features or peaks (also known as reflections and wavelets) corresponding to a number of subterranean reflectors or events. Due to the lateral offsets of the geophones in the array, the timeframe of the different traces for each geophone varies. Accordingly, these timeframes may be adjusted (e.g., normalized/correlated), and the adjusted traces may be stacked to yield data having a higher signal to noise ratio. This allows for better identification of subterranean events.
Such active seismic exploration requires the use of active seismic sources such as dynamite and/or vibrators. However, there are situations in which it may be desirable to avoid the use of active seismic sources. For instance, it may be difficult to utilize active seismic sources in rugged terrain or in populated areas. Further, use of such active seismic sources adds significantly to the cost of seismic imaging. In this regard, it may be preferable to utilize a passive seismic exploration method that does not utilize active seismic sources. In such a method, passive seismic sources may be utilized to generate seismic images of the subsurface. Such sources include, for example, micro-earthquakes and other natural ground vibration sources as well as surface generated noise such as that due to wind, cultural (oil pumps, quarry blasts, highway vehicles, etc.) or animal habitation or the like. Under the ocean, ice pack, waves, and animal noise may serve the same purpose.
One such theoretical passive seismic imaging system is sometimes referred to as “acoustic daylight imaging.” Acoustic daylight imaging is based on the theory that the seismic noise within the earth is analogous to daylight within the atmosphere. That is, both are incoherent, random fields of energy propagating in all directions, though their source mechanisms are different. In optical imaging, images are possible because the presence of an object modifies the ambient illumination by scattering the incident radiation (i.e., light). When the scattered light is focused by an optical lens onto a focal surface (e.g., retina or film), an image is created. Likewise, it is theorized that acoustic energy within the earth is reflected by objects within the earth. Accordingly, passive reflective imaging theorizes that an acoustic lens could be utilized to focus acoustic energy scattered by a subterranean object onto a focal surface. Simply stated, seismic vibrations/energy are considered similar to light and it is theorized that these vibrations will illuminate underground structures if properly focused. However, at present, no practical acoustic lens has been developed that allows for focusing on subterranean objects using ambient noise.